1. Field of the Invention
The invention relates to a method for synthesis of hierarchically ordered materials at multiple length scales using polyalkylene oxide triblock copolymers.
2. Description of Related Art
Nature abounds in hierarchical structures that are formed through highly coupled and often concurrent synthesis and assembly process over both molecular and long-range length scales. [Aksay, et al., Science, Vol. 273, 892 (1996)]. The existence of these hierarchical structures, such as abalones and diatoms, has both biological and evolutionary significance. The special architecture of the natural structures make them simultaneously hard, strong, and tough. It has thus been a long-sought goal to mimic the natural process responsible for these exquisite architectures using biomimetic strategies to control the structural organization and thereby produce useful materials with similar architecture.
Several approaches are currently available for the preparation of ordered structures at different length scales. For example, organic molecular templates can be used to form zeolitic structures with ordering lengths less than 3 nm [Bu, P., et al., Science, Vol. 278, 2080 (1997)]. Mesoporous materials with ordering lengths of 3-30 nm can be obtained using surfactants or amphiphilic block copolymers as the structure-directing agents [C. T. Kresge, et al., Nature, Vol. 359, 710 (1992); D. Zhao, et al., Science, Vol. 279, 548 (1998); P. Yang, et al., Nature, Vol. 399, 48 (1998); A. Firouzi, et al., J. Am. Chem. Soc. Vol.119, 9466 (1997); and S. H. Tolbert, et al., Science, Vol. 278, 264 (1997)].
Studies have shown the use of latex spheres affords macroporous materials with ordering lengths of 100 nm-1 xcexcm [O. D. Velev, et al., Nature, Vol. 389, 447 (1997); M. Antonietti, et al., Adv. Mater., Vol.10, 154 (1998); B. T. Holland, et al., Science, Vol. 281, 538 (1998); and J. E. G. J. Wijnhoven, et al., Science, Vol. 281, 802 (1998)]. Soft lithography has also been shown to make high-quality patterns and structures with lateral dimensions of about 30 nm to 500 xcexcm. [Y. Xia, et al., Angew. Chem. Int. Ed., Vol. 37, 550 (1998); E. Kim, et al., Adv. Mater. Vol. 8, 245 (1996); and C. Marzolin, et al., Adv. Mater. Vol.10, 571 (1998)].
Previous studies have shown use of micromolding to form patterned mesoporous materials [H. Yang, et al., Adv. Mater., Vol. 9, 811 (1997); and M. Trau, et al., Nature, Vol. 390, 674(1997)]. These studies, however, used acidic aqueous conditions to carry out the cooperative self-assembly, which is disadvantageous because of the poor processibility of the aqueous solutions. [Q. Huo, et al., Nature, Vol. 368, 317 (1994)]. The results of these studies were that either non-continuous films were formed or an electric field was needed to guide the mesophase growth, which required a non-conducting substrate [H. Yang, et al., supra; and M. Trau, et al., (1997), supra]. Studies have also shown the use of latex spheres to make disordered macro- and mesoporous silica [M. Antonietti, et al., supra].
Although previous studies addressed the synthesis of disordered mesoporous silica and alumina, using nonionic surfactants as the structure-directing agents and alkoxides as the inorganic sources, under aqueous media, the studies did not address large-pore mesoporous materials with vastly different composition, and nanocrystalline frameworks [Sayari, A., Chem. Mater. Vol. 8, 1840 (1996)].
Despite all of earlier efforts in nanostructuring materials, the fabrication of hierarchically ordered structures at multiple length scales has remained an experimental challenge. Such materials are important both for systematic fundamental study of structure-property relationships and for their technological promise in applications such as catalysis, selective separations, sensor arrays, waveguides, miniaturized electronic and magnetic devices, and photonic crystals with tunable band gaps [D. Zhao, et al., Adv. Mater. Vol.10, 1380 (1998) and M. E. Gimon-Kinsel, et al., Stud. Surf. Sci. Cata., Vol.117, 111(1998)].
Many applications for macro- and mesoporous metal oxides require structural ordering at multiple length scales. Thus, there exists a need for hierarchically ordered materials and a method for forming the materials which overcome or minimize the above mentioned problems and which have enormous potential for a variety of immediate and future industrial applications. A need also exists for forming the hierarchically ordered materials using low-cost, non-toxic, and biodegradable polyalkylene oxide block copolymers.
The present invention overcomes drawbacks of the foregoing methods to prepare mesoporous materials and mesoscopic structures having orders at multiple length scales, and provides heretofore unattainable materials having very desirable and widely useful properties. These materials are prepared by combining amphiphilic block copolymer with an inorganic metal compound, preferably by creating a sol-gel solution. The amphiphilic block copolymer species acts as a structure-directing agent for the inorganic metal compound in self-assembling systems. Pressure is applied to the combination, thus the block copolymer and inorganic metal compound are self-assembled and polymerized into a mesoscopically structured material.
In another embodiment of this invention, the combination, preferably a sol, is placed on a substrate and a mold is placed on top of the sol and the substrate. An effective amount of pressure is applied to the mold and substrate, the amount of pressure is preferably between about 1xc3x97105 to 2xc3x97105 Pa, whereby the sol-gel solution is compressed between the substrate and the mold. The mold is left in place, undisturbed, for at least about 12 hours to allow increased cross-linking and consolidation of the inorganic oxide network, thus the block copolymer and inorganic metal compound are self-assembled and polymerized into a mesoscopically structured material. The mold is then removed and the resulting patterned material is calcined to yield the patterned mesoscopically ordered porous material having a multiple length scale. Calcination occurs at 300xc2x0 C. to 600xc2x0 C., preferably at about 400xc2x0 C.-450xc2x0 C. Unlike previous mesoscopically ordered materials, the materials described in this invention can be produced with multiple length scales on the order of approximately 10 nm and 100 nm.
In accordance with a further embodiment of the invention mesoporous materials and mesoscopic structures having orders at multiple length scales are synthesized. Synthesis is carried out by contacting a mold, which ends have been cut, with a substrate. A mold is filled with a latex colloidal suspension, whereby an array is formed within the mold. An amphiphilic block copolymer is then combined with an inorganic metal compound, preferably by creating a sol, and the mold is filled with the sol whereby the block copolymer and inorganic metal compound are self-assembled and polymerized into a mesoscopically structured material exhibiting multiple structural ordering length scales on the order of approximately 10, 100, and 1000 nm.